Integrated circuits, especially those used in computer memory arrays, are often designed in repetitive patterns, employing a unit cell duplicated over a large area of a semiconductor chip. Manufacturing yields for such dense circuitry tend to be low, because there is a greater probability that particles or other defects will coincide with electrically active devices or interconnecting wires, rendering them inoperable. One way to solve this problem entails building in redundant cells that can be substituted for failed cells discovered during electrical testing. A laser beam can accomplish this substitution step by severing certain electrical connections, while leaving others intact.
This error correction process is most efficiently executed at the wafer level, prior to dividing a wafer into individual chips. In a typical memory repair system the full wafer is transported on a robotically controlled stage beneath a laser optics assembly, which is programmed to trigger a laser pulse when the laser beam axis aligns with the desired connection point, or link. The velocity of the laser beam axis on the wafer relative to the link (also known as the link run velocity) and the accuracy of the laser beam triggering system define the quality and throughput of such link processing platforms.
At the present time, laser repair systems are capable of processing 100,000 links per second, at link run velocities up to 210 mm/sec, on devices spaced approximately 2 microns apart. Lasers suitable for memory repair are presently available with pulse repetition frequencies (PRFs) up to 150 kHz, or 150,000 pulses per second. Laser advances continue to increase the pulse repetition frequency, and lasers with several hundred kHz PRF are anticipated. Electro Scientific Industries, Inc., the assignee of this patent application, offers a Model 9850 dual beam link processing system that can double the laser PRF in a particular operational mode. It is desirable to process link runs at a velocity that is the product of the laser PRF and the link pitch. When this velocity exceeds the maximum capability of the system, the link run must be performed at a slower speed. Thus, laser beam pulses can be triggered at a much faster rate than the speed at which the laser beam axis can advance from one target to the next. This difference affords an opportunity to increase the link run velocity considerably.